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How to kill knotweed: Let slip the bugs of war

Knotweed can pierce tarmac and even concrete, causing serious damage to infrastructure

Hugh Williamson / Alamy Stock Photo

By Stephanie Pain

WE ARE under attack. An alien is overrunning our gardens and breaking into our homes – cracking concrete foundations, smashing through brickwork and rearing up between floorboards. The invader is Japanese knotweed, and it is no ordinary weed. This plant, it’s fair to say, is one of the most hated in the world.

And the headline-grabbing horror stories of infested homes and plummeting property prices are just part of it. Out in the wild, knotweed is choking waterways, undermining flood defences and smothering native habitats across large swathes of Europe and North America. The plant has even found its way to Australia and New Zealand. “Outside its native range, knotweed has the biodiversity value of concrete,” says entomologist Dick Shaw. “Nothing lives on it. Nothing eats it.”

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Shaw is mounting a counter-offensive. This spring, I visited his lab in a leafy corner of London’s stockbroker belt. Outside, all over the country, pink shoots of Japanese knotweed were bursting through soil, tarmac and concrete and heading skywards. The bamboo-like stems can grow as much as a metre a month. Yet the ones in Shaw’s lab weren’t growing much at all. They were stunted and draped with thin waxy white ribbons.

The wax, Shaw explained, is secreted by small sap-sucking insects called psyllids. He points to the new leaves at the tips of the shoots. In every case, …